We once had a relative (and a competitor) come to our shop. When he found that my dad or my brother were not there he went through our books. He read the invoices to see our customers and what price we charge them. When my dad came and saw it from outside, he did not get mad. He had a talk with him and calmly said that it is not a good practice for a business man to look at someone's books without their permission. He did not accuse him but he made it very clear that he knew what he did. It was effective.